The purpose of this project is to study the pharmacologic activity and the biochemical mode of action of new antitumor agents of the pyrimidine nucleoside class. Present studies are focused on 5-azacytidine and 5,6-dihydro-5-azacytidine, a reduced analog of the latter with greater chemical stability than the parent compound. Topics of current interest are combination chemotherapy studies both in vitro and in vivo and studies designed to test whether the dihydro derivative is a pro-drug of the parent or acting in its own right.